Light-emitting element, light-emitting device, electronic device, lighting device, and pyrene-based compound

ABSTRACT

A highly efficient light-emitting element capable of providing a plurality of emission colors is provided, which does not easily deteriorate and can minimize a decrease in external quantum efficiency even when a light-emitting layer has a stacked structure. A light-emitting device, an electronic device, and a lighting device which have low power consumption and long lifetime are provided. A light-emitting element includes a plurality of light-emitting layers stacked between a pair of electrodes. The light-emitting layers each contain a host material and a guest material. The guest materials of the light-emitting layers are substances which have different HOMO levels but have substantially the same LUMO levels and emit light of different colors. A light-emitting device, an electronic device, and a lighting device are fabricated using the light-emitting element.

This application is a continuation of U.S. application Ser. No.15/152,091, filed on May 11, 2016 which is a continuation of copendingU.S. application Ser. No. 13/644,275, filed on Oct. 4, 2012 (now U.S.Pat. No. 9,343,681 issued May 17, 2016) which are all incorporatedherein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

One embodiment of the present invention relates to light-emittingelements having a plurality of emission colors. One embodiment of thepresent invention also relates to light-emitting devices, electronicdevices, and lighting devices. One embodiment of the present inventionfurther relates to novel pyrene-based compounds.

2. Description of the Related Art

In recent years, research and development have been extensivelyconducted on light-emitting elements using electroluminescence (EL). Ina basic structure of such a light-emitting element, a layer containing alight-emitting substance is interposed between a pair of electrodes. Byapplying voltage to this element, light emission from the light-emittingsubstance can be obtained.

Since such a light-emitting element is of self-light-emitting type, itis considered that the light-emitting element has advantages over aliquid crystal display in that visibility of pixels is high, backlightis not required, and so on and is therefore suitable as flat paneldisplay elements. In addition, it is also a great advantage that thelight-emitting element can be manufactured as a thin and lightweightelement. Furthermore, very high speed response is also one of thefeatures of such elements.

Furthermore, since such light-emitting elements can be formed in theform of a film, they make it possible to provide planar light emissioneasily. Therefore, large-area elements using planar light emission canbe easily formed. This feature is difficult to obtain with point lightsources typified by incandescent lamps and LEDs or linear light sourcestypified by fluorescent lamps. Thus, light-emitting elements also havegreat potential as planar light sources which can be applied to lightingdevices and the like.

Light-emitting elements utilizing electroluminescence are broadlyclassified according to whether they include an organic compound or aninorganic compound as a light-emitting substance. In the case where anorganic compound is used as a light-emitting substance, application ofvoltage to a light-emitting element causes electrons and holes to beinjected into a layer containing the light-emitting organic compoundfrom a pair of electrodes, whereby current flows. The carriers(electrons and holes) are recombined, and thus the light-emittingorganic compound is excited. The light-emitting organic compound returnsto a ground state from the excited state, thereby emitting light. Notethat the excited state of an organic compound can be a singlet excitedstate and a triplet excited state, and light emission from the singletexcited state (S*) is referred to as fluorescence, and light emissionfrom the triplet excited state (T*) is referred to as phosphorescence.

A function as a light-emitting layer of a light-emitting element formedusing a light-emitting organic compound can be achieved with thelight-emitting organic compound alone. However, a method for forming alight-emitting layer in which a light-emitting organic compound isdispersed in a matrix of another substance is also employed for thepurpose of preventing concentration quenching of the light-emittingorganic compound, for example. Note that a substance serving as a matrixis called a host material, and a substance dispersed in the matrix iscalled a guest material.

In that case, carriers (electrons and holes) injected from bothelectrodes are recombined in the host material of the light-emittinglayer, and the guest material receives the energy and emits light.Therefore, it is known that light emission with high luminance and highcolor purity can be achieved.

In addition, light-emitting elements which emit white light haverecently been employed for lighting purposes. In such a case, whitelight emission can be achieved with the use of a plurality oflight-emitting materials. However, a light-emitting layer containingplural kinds of light-emitting materials causes problems such as achange in chromaticity and a decrease in external quantum efficiency,due to energy transfer between the light-emitting materials.

Against these problems, it has been proposed to stack a plurality oflight-emitting layers, each containing a different light-emittingmaterial (see, for example, Reference 1). However, in that case, aproblem is a high possibility of deterioration because of chargeaccumulation at the interface between the stacked layers, for example.

REFERENCE

[Reference 1] Japanese Published Patent Application No. 2006-269232

SUMMARY OF THE INVENTION

One embodiment of the present invention provides a highly efficientlight-emitting element capable of providing a plurality of emissioncolors, which does not easily deteriorate and can minimize a decrease inexternal quantum efficiency even when a light-emitting layer has astacked structure. One embodiment of the present invention also providesa novel pyrene-based compound suitable for the light-emitting layer ofthe above light-emitting element. One embodiment of the presentinvention also provides a highly efficient light-emitting element byusing the novel pyrene-based compound. One embodiment of the presentinvention also provides a light-emitting device, an electronic device,and a lighting device which have low power consumption and longlifetime, by using the light-emitting element containing the novelpyrene-based compound.

One embodiment of the present invention is a light-emitting elementincluding a plurality of light-emitting layers stacked between a pair ofelectrodes. The light-emitting layers each contain a host material and aguest material. The guest materials of the light-emitting layers aresubstances which have different HOMO levels but have substantially thesame LUMO levels and emit light of different colors. Note that LUMOlevels within a range of ±0.2 eV, preferably within a range of ±0.1 eV,are regarded as substantially the same LUMO levels.

Note that different guest materials are used for the plurality oflight-emitting layers, and as the different guest materials, materialshaving different HOMO levels but having substantially the same LUMOlevels are used. Accordingly, a stack-type light-emitting layer can beformed which has a hole-trapping property at the interface between thestacked light-emitting layers but is unlikely to block electrontransfer. Therefore, a light-emitting element having high emissionefficiency and long lifetime can be formed.

In the above light-emitting element, the host materials of thelight-emitting layers are preferably a common (in other words, the same)material, which is preferably a bipolar material. The use of the commonbipolar material as the host materials can reduce the influence of thehost materials on the carrier-transport property even in thelight-emitting layer having a stacked structure and can facilitateelement design.

The common host material used for the light-emitting layers preferablycontains condensed aromatic hydrocarbon in addition to being bipolar.This is because a material containing condensed aromatic hydrocarbon hasa higher molecular weight and better thermophysical properties. With theuse of the material containing condensed aromatic hydrocarbon, alight-emitting element having high heat resistance can be formed. Notethat examples of the material containing condensed aromatic hydrocarboninclude materials containing anthracene, triphenylene, pyrene,phenanthrene, or fluoranthene.

In the above light-emitting element, the guest materials used for thelight-emitting layers are preferably pyrene-based compounds havingdifferent structures. The pyrene-based compounds are preferablypyrenediamine compounds, particularly, pyrene-1,6-diamine compounds. Inaddition, the host material is preferably a material containinganthracene, particularly, a material containing anthracene and having noamine skeleton.

Another embodiment of the present invention is a novel pyrene-basedcompound which can be used as a guest material in each light-emittinglayer of the above light-emitting element and is represented by thefollowing general formula (G1).

In the formula, Ar represents a substituted or unsubstituted aryl grouphaving 6 to 10 carbon atoms, α represents a substituted or unsubstitutedphenylene group, and R¹ to R¹² separately represent a hydrogen atom oran alkyl group having 1 to 4 carbon atoms. Further, R¹³ to R²⁰separately represent a hydrogen atom or an alkyl group having 1 to 6carbon atoms. Further, n is 0 or 1.

Another embodiment of the present invention is a novel pyrene-basedcompound represented by the following general formula (G2).

In the formula, Ar represents a substituted or unsubstituted aryl grouphaving 6 to 10 carbon atoms, and R¹ to R¹² separately represent ahydrogen atom or an alkyl group having 1 to 4 carbon atoms. Further, R¹³to R²⁰ separately represent a hydrogen atom or an alkyl group having 1to 6 carbon atoms.

Another embodiment of the present invention is a novel pyrene-basedcompound represented by the following general formula (G3).

In the formula, R¹ to R¹² and R²¹ to R²⁵ separately represent a hydrogenatom or an alkyl group having 1 to 4 carbon atoms.

Another embodiment of the present invention is a novel pyrene-basedcompound represented by the following structural formula (100).

The novel pyrene-based compounds which are embodiments of the presentinvention emit light of a blue-green color with high color purity.Therefore, a light-emitting element including a plurality oflight-emitting layers stacked between a pair of electrodes, in which thelight-emitting layers each contain a host material and a guest materialand plural kinds of pyrene-based compounds are used as the guestmaterials of the light-emitting layers, can provide light of emissioncolors of all the guest materials at the same time, when thelight-emitting layers are formed by using as a guest material at leastone of above-described novel pyrene-based compounds which areembodiments of the present invention and by using as another guestmaterial another pyrene-based compound having a different HOMO level buthaving substantially the same LUMO level.

When a light-emitting layer containing an orange light-emittingsubstance is further stacked over the light-emitting layer having astacked structure containing different guest materials in thelight-emitting element of one embodiment of the present invention, thelight-emitting element can emit excellent white light.

Further, the present invention includes, in its scope, electronicdevices and lighting devices including light-emitting devices as well aslight-emitting devices including light-emitting elements. Thelight-emitting device in this specification refers to an image displaydevice, a light-emitting device, and a light source (e.g., a lightingdevice). In addition, the light-emitting device includes all thefollowing modules: a module in which a connector, such as a flexibleprinted circuit (FPC), a tape automated bonding (TAB) tape, or a tapecarrier package (TCP), is attached to a light-emitting device; a modulein which a printed wiring board is provided at the end of a TAB tape ora TCP; and a module in which an integrated circuit (IC) is directlymounted on a light-emitting device by a chip-on-glass (COG) method.

One embodiment of the present invention can provide a highly efficientlight-emitting element capable of providing a plurality of emissioncolors, which does not easily deteriorate and can minimize a decrease inexternal quantum efficiency even when a light-emitting layer has astacked structure. One embodiment of the present invention can alsoprovide a novel pyrene-based compound suitable for the light-emittinglayer of the above light-emitting element. One embodiment of the presentinvention can also provide a highly efficient light-emitting element byusing the novel pyrene-based compound. One embodiment of the presentinvention can also provide a light-emitting device, an electronicdevice, and a lighting device which have low power consumption and longlifetime, by using the light-emitting element containing the novelpyrene-based compound.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B illustrate a structure of a light-emitting element.

FIGS. 2A and 2B each illustrate a structure of a light-emitting element.

FIGS. 3A and 3B illustrate a light-emitting device.

FIGS. 4A to 4D illustrate electronic devices.

FIGS. 5A to 5C illustrate an electronic device.

FIG. 6 illustrates lighting devices.

FIGS. 7A and 7B show ¹H-NMR charts of a pyrene-based compoundrepresented by a structural formula (100).

FIGS. 8A and 8B show ultraviolet-visible absorption spectra and emissionspectra of the pyrene-based compound represented by the structuralformula (100).

FIGS. 9A and 9B show ¹H-NMR charts of a pyrene-based compoundrepresented by a structural formula (108).

FIGS. 10A and 10B show ultraviolet-visible absorption spectra andemission spectra of the pyrene-based compound represented by thestructural formula (108).

FIG. 11 illustrates a light-emitting element 1.

FIG. 12 shows luminance-current efficiency characteristics of thelight-emitting element 1.

FIG. 13 shows voltage-current characteristics of the light-emittingelement 1.

FIG. 14 is a chromaticity diagram showing the chromaticity of thelight-emitting element 1.

FIG. 15 shows an emission spectrum of the light-emitting element 1.

FIG. 16 shows reliability of the light-emitting element 1.

FIG. 17 illustrates a light-emitting element 2.

FIG. 18 shows luminance-current efficiency characteristics of thelight-emitting element 2.

FIG. 19 is a chromaticity diagram showing the chromaticity of thelight-emitting element 2.

FIG. 20 shows an emission spectrum of the light-emitting element 2.

FIG. 21 shows results of LC-MS measurement of a heterocyclic compoundrepresented by a structure formula (100).

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described below withreference to the drawings. Note that the present invention is notlimited to the following description, and various changes andmodifications can be made without departing from the spirit and scope ofthe invention. Therefore, the present invention should not be construedas being limited to the description in the following embodiments.

Embodiment 1

In this embodiment, a light-emitting element which is one embodiment ofthe present invention will be described with reference to FIGS. 1A and1B.

In the light-emitting element described in this embodiment, asillustrated in FIG. 1A, an EL layer 102 including a light-emitting layer113 is provided between a pair of electrodes (a first electrode (anode)101 and a second electrode (cathode) 103), and the EL layer 102 includesa hole-injection layer 111, a hole-transport layer 112, anelectron-transport layer 114, an electron-injection layer 115, a chargegeneration layer (E) 116, and the like in addition to the light-emittinglayer 113.

Application of a voltage to such a light-emitting element causes holesinjected from the first electrode 101 side and electrons injected fromthe second electrode 103 side to recombine in the light-emitting layer113 and a substance contained in the light-emitting layer 113 to beraised to an excited state. Then, light is emitted when the substance inthe excited state returns to the ground state.

Note that the light-emitting layer 113 in the EL layer 102 has a stackedstructure including a plurality of layers. For example, when having astacked structure of two layers, the light-emitting layer 113 includes afirst light-emitting layer 113 a and a second light-emitting layer 113 bas illustrated in FIG. 1B.

The first light-emitting layer 113 a and the second light-emitting layer113 b are formed so as to have substantially the same LUMO levels(LUMO(a)≈LUMO(b)) but have different HOMO levels (HOMO(a)≠HOMO(b)). Inthe case where the HOMO level of the first light-emitting layer 113 a(HOMO(a)) is deeper than the HOMO level of the second light-emittinglayer 113 b (HOMO(b)) as illustrated in FIG. 1B, a hole-trappingproperty is accordingly acquired, and thus a recombination region forholes and electrons can be confined within the light-emitting layer.Therefore, the light-emitting element can have higher emissionefficiency than a conventional light-emitting element. Thus, a guestmaterial contained in the anode side of the light-emitting layer (thefirst light-emitting layer) preferably emits light of a shorterwavelength than that of a guest material contained in the cathode side(the second light-emitting layer).

In the case where the first light-emitting layer 113 a and the secondlight-emitting layer 113 b are formed so as to have substantially thesame LUMO levels (LUMO(a)≈LUMO(b)) but have different HOMO levels(HOMO(a)≠HOMO(b)), the first light-emitting layer 113 a and the secondlight-emitting layer 113 b are formed using the same host material anddifferent guest materials. Note that even when the light-emitting layers(the first light-emitting layer 113 a and the second light-emittinglayer 113 b) emit light of different colors, holes are trapped at theinterface between the first light-emitting layer 113 a and the secondlight-emitting layer 113 b because the different guest materials areused, and thus light of different colors can be emitted at the sametime. This is a great advantage in, for example, obtaining white lightin combination with another light-emitting layer because color renderingproperties can be improved.

The hole-injection layer 111 in the EL layer 102 is a layer containing asubstance having a high hole-transport property and an acceptorsubstance. When electrons are extracted from the substance having a highhole-transport property owing to the acceptor substance, holes aregenerated. Thus, holes are injected from the hole-injection layer 111into the light-emitting layer 113 through the hole-transport layer 112.

The charge generation layer (E) 116 is a layer containing a substancehaving a high hole-transport property and an acceptor substance.Electrons are extracted from the substance having a high hole-transportproperty owing to the acceptor substance, and the extracted electronsare injected from the electron-injection layer 115 having anelectron-injection property into the light-emitting layer 113 throughthe electron-transport layer 114.

A specific example in which the light-emitting element described in thisembodiment is manufactured is described below.

As the first electrode (anode) 101 and the second electrode (cathode)103, a metal, an alloy, an electrically conductive compound, a mixturethereof, or the like can be used. Specifically, indium oxide-tin oxide(ITO: indium tin oxide), indium oxide-tin oxide containing silicon orsilicon oxide, indium oxide-zinc oxide (indium zinc oxide), indium oxidecontaining tungsten oxide and zinc oxide, gold (Au), platinum (Pt),nickel (Ni), tungsten (W), chromium (Cr), molybdenum (Mo), iron (Fe),cobalt (Co), copper (Cu), palladium (Pd), or titanium (Ti) can be used.In addition, an element belonging to Group 1 or Group 2 of the periodictable, for example, an alkali metal such as lithium (Li) or cesium (Cs),an alkaline earth metal such as magnesium (Mg), calcium (Ca), orstrontium (Sr), an alloy containing such an element (e.g., MgAg orAlLi), a rare earth metal such as europium (Eu) or ytterbium (Yb), analloy containing such an element, graphene, or the like can be used. Thefirst electrode (anode) 101 and the second electrode (cathode) 103 canbe formed by, for example, a sputtering method, an evaporation method(including a vacuum evaporation method), or the like.

Examples of the substance having a high hole-transport property which isused for the hole-injection layer 111, the hole-transport layer 112, andthe charge generation layer (E) 116 include aromatic amine compoundssuch as 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation:NPB or α-NPD),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD), 4,4′,4″-tris(carbazol-9-yl)triphenylamine(abbreviation: TCTA), 4,4′,4″-tris(N,N′-diphenylamino)triphenylamine(abbreviation: TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA), and4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB),3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1),3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2), and3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1). Other examples include carbazole derivativessuch as 4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP),1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB), and9-[4-(10-phenyl-9-anthracenyl)phenyl]-9H-carbazole (abbreviation: CzPA).The substances mentioned here are mainly substances that have a holemobility of 10⁻⁶ cm²/Vs or more. Note that other than these substances,any substance that has a property of transporting more holes thanelectrons may be used.

Still other examples include high molecular compounds such aspoly(N-vinylcarbazole) (abbreviation: PVK), poly(4-vinyltriphenylamine)(abbreviation: PVTPA),poly[N-(4-{N′-[4-(4-diphenylamino)phenyl]phenyl-N′-phenylamino}phenyl)methacrylamide](abbreviation: PTPDMA), andpoly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] (abbreviation:Poly-TPD).

Further, examples of the acceptor substance that is used for thehole-injection layer 111 and the charge generation layer (E) 116 includeoxides of transition metals, oxides of metals belonging to Groups 4 to 8of the periodic table, and the like. Specifically, molybdenum oxide isparticularly preferable. Alternatively, an organic compound may be usedas the acceptor substance.

The light-emitting layer 113 has a stacked structure, and thelight-emitting layers each contain a host material and a guest material.Note that the host materials of the light-emitting layers are preferablya common (in other words, the same) material having a higher tripletexcitation energy than the guest materials, which is preferably abipolar material. The material further preferably contains condensedaromatic hydrocarbon in addition to being bipolar. Specific examplesinclude materials containing anthracene, triphenylene, pyrene,phenanthrene, or fluoranthene.

The guest materials of the light-emitting layers are substances whichhave different HOMO levels but have substantially the same LUMO levelsand emit light of different colors. The guest materials used for thelight-emitting layers are preferably pyrene-based compounds havingdifferent structures. The pyrene-based compounds are preferablypyrenediamine compounds, particularly, pyrene-1,6-diamine compounds.This is because a change of the amine skeleton in the pyrenediaminecompound can make a change to the emission color and the HOMO levelwhile the LUMO level originating from pyrene is kept substantially thesame. Furthermore, the host material is preferably a material containinganthracene, particularly, a material containing anthracene and having noamine skeleton. Such a material has substantially the same LUMO level asthe pyrenediamine compound, which allows smooth electron transfer. Inaddition, it has a deeper HOMO level than the pyrenediamine compound;therefore, hole trapping works effectively. With such a structure, anelement having low drive voltage and high emission efficiency can bemanufactured. Furthermore, holes are unlikely to pass through thelight-emitting layer; therefore, the luminance is unlikely to decreaseat the time of constant-current driving even when the electron-transportproperty or the electron-injection property of the electron-transportlayer or the cathode decreases over time. Thus, a light-emitting elementhaving long lifetime can be obtained.

Note that embodiments of pyrene-based compounds which can be used as theguest materials will be described later in Embodiment 2 and Examples 1to 3. Note that in the light-emitting layer having a stacked structurein the light-emitting element of this embodiment, different materialsare used as guest materials of the light-emitting layers, and at leastone of the pyrene-based compounds described in Embodiment 2 is used.

The electron-transport layer 114 is a layer that contains a substancehaving a high electron-transport property. For the electron-transportlayer 114, it is possible to use a metal complex such as Alq₃,tris(4-methyl-8-quinolinolato)aluminum (abbreviation: Almq₃),bis(10-hydroxybenzo[h]quinolinato)beryllium (abbreviation: BeBq₂), BAlq,Zn(BOX)₂, or bis[2-(2-hydroxyphenyl)benzothiazolato]zinc (abbreviation:Zn(BTZ)₂). Alternatively, it is possible to use a heteroaromaticcompound such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),3-(4-tert-butylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-triazole(abbreviation: TAZ),3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole(abbreviation: p-EtTAZ), bathophenanthroline (abbreviation: BPhen),bathocuproine (abbreviation: BCP), or4,4′-bis(5-methylbenzoxazol-2-yl)stilbene (abbreviation: BzOs). Furtheralternatively, it is possible to use a high molecular compound such aspoly(2,5-pyridine-diyl) (abbreviation: PPy),poly[(9,9-dihexylfluorene-2,7-diyl)-co-(pyridine-3,5-diyl)](abbreviation: PF-Py), orpoly[(9,9-dioctylfluorene-2,7-diyl)-co-(2,2′-bipyridine-6,6′-diyl)](abbreviation: PF-BPy). The substances mentioned here are mainlysubstances that have an electron mobility of 10⁻⁶ cm²/Vs or more. Notethat other than these substances, any substance that has a property oftransporting more electrons than holes may be used for theelectron-transport layer.

The electron-transport layer is not limited to a single layer, and maybe a stack of two or more layers containing any of the above substances.

The electron-injection layer 115 is a layer that contains a substancehaving a high electron-injection property. Examples of the substancethat can be used for the electron-injection layer 115 include alkalimetals, alkaline earth metals, and compounds thereof, such as lithiumfluoride (LiF), cesium fluoride (CsF), calcium fluoride (CaF₂), andlithium oxide (LiO_(x)), and rare earth metal compounds, such as erbiumfluoride (ErF₃). Alternatively, the above-mentioned substances forforming the electron-transport layer 114 can be used.

Alternatively, a composite material in which an organic compound and anelectron donor (a donor) are mixed may be used for theelectron-injection layer 115. Such a composite material, in whichelectrons are generated in the organic compound by the electron donor,has high electron-injection and electron-transport properties. Theorganic compound here is preferably a material excellent in transportingthe generated electrons, and specifically any of the above substances(such as metal complexes and heteroaromatic compounds) for theelectron-transport layer 114 can be used. As the electron donor, asubstance showing an electron-donating property with respect to theorganic compound may be used. Specifically, alkali metals, alkalineearth metals, and rare earth metals are preferable, and examples includelithium, cesium, magnesium, calcium, erbium, and ytterbium. Any ofalkali metal oxides and alkaline earth metal oxides is preferable,examples of which are lithium oxide, calcium oxide, barium oxide, andthe like, and a Lewis base such as magnesium oxide or an organiccompound such as tetrathiafulvalene (abbreviation: TTF) can be used.

Note that the hole-injection layer 111, the hole-transport layer 112,the light-emitting layer 113, the electron-transport layer 114, theelectron-injection layer 115, and the charge generation layer (E) 116which are mentioned above can each be formed by a method such as anevaporation method (including a vacuum evaporation method), an inkjetmethod, or a coating method. In the case where the light-emitting layer113 has a stacked structure, a layer formed by an evaporation method anda layer formed by an inkjet method may be combined.

In the above-described light-emitting element, a current flows due to apotential difference generated between the first electrode 101 and thesecond electrode 103, and holes and electrons recombine in the EL layer102, so that light is emitted. Then, this light emission is extracted tothe outside through either the first electrode 101 or the secondelectrode 103 or both. Therefore, either the first electrode 101 or thesecond electrode 103, or both, is an electrode having alight-transmitting property.

Note that the light-emitting element described in this embodiment is oneembodiment of the present invention and is an example of alight-emitting element which includes a light-emitting layer havingsubstantially the same LUMO levels in spite of having different HOMOlevels. In such a light-emitting element, a recombination region forholes and electrons can be confined within the light-emitting layerowing to the hole-trapping property acquired by providing different HOMOlevels in the light-emitting layer. Thus, a light-emitting elementhaving higher efficiency than a conventional light-emitting element canbe obtained. Further, as a light-emitting device including the abovelight-emitting element, a passive matrix light-emitting device, anactive matrix light-emitting device, and the like can be manufactured.Each of the above light-emitting devices is included in the presentinvention.

Note that there is no particular limitation on the structure of a TFT inthe case of manufacturing the active matrix light-emitting device. Forexample, a staggered TFT or an inverted staggered TFT can be used asappropriate. Further, a driver circuit formed over a TFT substrate maybe formed using both an n-type TFT and a p-type TFT or either an n-typeTFT or a p-type TFT. Furthermore, there is also no particular limitationon the crystallinity of a semiconductor film used for the TFT. Forexample, an amorphous semiconductor film, a crystalline semiconductorfilm, an oxide semiconductor film, or the like can be used.

Note that the structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

Embodiment 2

In this embodiment, novel pyrene-based compounds which are embodimentsof the present invention and can be used in the light-emitting elementdescribed in Embodiment 1 will be described.

One embodiment of the present invention is a pyrene-based compoundhaving a structure represented by the following general formula (G1).

In the general formula (G1), Ar represents a substituted orunsubstituted aryl group having 6 to 10 carbon atoms, a represents asubstituted or unsubstituted phenylene group, and R¹ to R¹² separatelyrepresent a hydrogen atom or an alkyl group having 1 to 4 carbon atoms.Further, R¹³ to R²⁰ separately represent a hydrogen atom or an alkylgroup having 1 to 6 carbon atoms. Further, n is 0 or 1.

Here, specific examples of a include a phenylene group and a phenylenegroup substituted by one or more alkyl groups each having 1 to 4 carbonatoms.

Note that n is preferably equal to 0 in the above general formula (G1)for easier synthesis. Thus, another embodiment of the present inventionis a pyrene-based compound including a structure represented by thefollowing general formula (G2).

In the general formula (G2), Ar represents a substituted orunsubstituted aryl group having 6 to 10 carbon atoms, and R¹ to R¹²separately represent a hydrogen atom or an alkyl group having 1 to 4carbon atoms. Further, R¹³ to R²⁰ separately represent a hydrogen atomor an alkyl group having 1 to 6 carbon atoms.

Among pyrene-based compounds including the structure represented by thegeneral formula (G2), a pyrene-based compound represented by thefollowing general formula (G3) is preferable.

In the general formula (G3), R¹ to R¹² and R²¹ to R²⁵ separatelyrepresent a hydrogen atom or an alkyl group having 1 to 4 carbon atoms.

In the general formulae (G1) to (G3), specific examples of the alkylgroup having 1 to 4 carbon atoms for any of R¹ to R¹² include a methylgroup, an ethyl group, a propyl group, an isopropyl group, a butylgroup, a sec-butyl group, an isobutyl group, a tert-butyl group, and thelike. Specific examples of the alkyl group having 1 to 6 carbon atomsfor any of R¹³ to R²⁰ include a methyl group, an ethyl group, a propylgroup, an isopropyl group, a butyl group, a sec-butyl group, an isobutylgroup, a tert-butyl group, a pentyl group, an isopentyl group, a hexylgroup, a cyclohexyl group, and the like.

Next, specific structural formulae of the pyrene-based compounds whichare embodiments of the present invention are shown (the followingstructural formulae (100) to (114)). Note that the present invention isnot limited thereto.

A variety of reactions can be applied to methods of synthesizing theabove-described novel pyrene-based compounds which are embodiments ofthe present invention. For example, the pyrene-based compound which isone embodiment of the present invention and represented by the generalformula (G1) can be synthesized by synthesis reactions described below.Note that methods of synthesizing the novel pyrene-based compounds whichare embodiments of the present invention are not limited to thesynthesis methods described below.

Method of Synthesizing Pyrene-Based Compound Represented by GeneralFormula (G1)

An example of a method of synthesizing the pyrene-based compoundrepresented by the following general formula (G1) will be described.

As illustrated in the following synthesis scheme (A-1), a halide of acarbazole derivative (a1) and an aryl compound having an amine (a2) arecoupled, whereby an amine derivative (a3) can be obtained.

Note that in the synthesis scheme (A-1), Ar represents a substituted orunsubstituted aryl group having 6 to 10 carbon atoms. Further, R¹ to R¹²separately represent a hydrogen atom or an alkyl group having 1 to 4carbon atoms. Further, c represents a substituted or unsubstitutedphenylene group. Further, n is 0 or 1. Further, X¹ represents a halogen,preferably bromine or iodine, more preferably iodine, which has highreactivity.

In the synthesis scheme (A-1), the aryl compound having an amine (aprimary arylamine compound or a secondary arylamine compound) and thehalide of a carbazole derivative may be coupled by a variety ofsynthesis methods under a variety of reaction conditions. As an examplethereof, a synthesis method using a metal catalyst in the presence of abase (e.g., the Hartwig-Buchwald reaction or the Ullmann reaction) canbe employed.

Then, the case where the Hartwig-Buchwald reaction is performed in thesynthesis scheme (A-1) is described. As the metal catalyst, a palladiumcatalyst can be used, and as the palladium catalyst, a mixture of apalladium complex and a ligand thereof can be used. Specific examples ofthe palladium complex include bis(dibenzylideneacetone)palladium(0),palladium(II) acetate, and the like. Examples of the ligand includetri(tert-butyl)phosphine, tri(n-hexyl)phosphine, tricyclohexylphosphine,1,1-bis(diphenylphosphino)ferrocene (abbreviation: DPPF), and the like.Examples of a substance which can be used as the base include organicbases such as sodium tert-butoxide, inorganic bases such as potassiumcarbonate, and the like.

Note that the reaction is preferably performed in a solution. Examplesof a solvent which can be used include toluene, xylene, benzene, and thelike. However, the catalyst, ligand, base, and solvent which can be usedare not limited thereto. In addition, the reaction is preferablyperformed under an inert atmosphere of nitrogen, argon, or the like.

Next, the case where the Ullmann reaction is performed in the synthesisscheme (A-1) is described. As the metal catalyst, a copper catalyst canbe used, and specifically, copper(I) iodide or copper(II) acetate can beused. Examples of a substance which can be used as the base includeinorganic bases such as potassium carbonate.

This reaction is also preferably performed in a solution. Examples of asolvent which can be used include1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone (DMPU), toluene,xylene, benzene, and the like. However, the catalyst, base, and solventwhich can be used are not limited thereto. In addition, the reaction ispreferably performed under an inert atmosphere of nitrogen, argon, orthe like.

Note that a solvent having a high boiling point such as DMPU or xyleneis preferably used because, in the Ullmann reaction, an object can beobtained in a shorter time and in a higher yield when the reactiontemperature is 100° C. or higher. In particular, DMPU is more preferablebecause the reaction temperature is more preferably 150° C. or higher.

Next, a synthesis scheme (A-2) is shown. As illustrated in the followingsynthesis scheme (A-2), the amine derivative (a3) and a halogenatedpyrene derivative (a4) are coupled, whereby the amine derivativerepresented by the general formula (G1) can be obtained.

Note that in the synthesis scheme (A-2), Ar represents a substituted orunsubstituted aryl group having 6 to 10 carbon atoms. Further, R¹ to R¹²separately represent a hydrogen atom or an alkyl group having 1 to 4carbon atoms. Further, R¹³ to R²⁰ separately represent a hydrogen atomor an alkyl group having 1 to 6 carbon atoms. Further, α represents asubstituted or unsubstituted phenylene group. Further, n is 0 or 1.Further, X² and X³ each represent a halogen. Note that the halogen ispreferably bromine or iodine, more preferably iodine, which has highreactivity.

In the synthesis scheme (A-2), two equivalents of the amine derivative(a3) are reacted with the halogenated pyrene derivative (a4).

Also in the synthesis scheme (A-2), the aryl compound having an amine (aprimary arylamine compound or a secondary arylamine compound) and thearyl compound having a halogen group may be coupled by a variety ofsynthesis methods under a variety of reaction conditions, as in thesynthesis scheme (A-1). As an example thereof, a synthesis method usinga metal catalyst in the presence of a base (e.g., the Hartwig-Buchwaldreaction or the Ullmann reaction) can be employed.

One example of the method of synthesizing the pyrene-based compound (G1)which is one embodiment of the present invention is described above;however, the present invention is not limited thereto and any othersynthesis method may be employed.

With the use of the pyrene-based compound that is one embodiment of thepresent invention, a light-emitting element, a light-emitting device, anelectronic device, or a lighting device having high emission efficiencycan be obtained. Furthermore, a light-emitting element, a light-emittingdevice, an electronic device, or a lighting device having low powerconsumption can be obtained.

The structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

Embodiment 3

In this embodiment, as one embodiment of the present invention, alight-emitting element (hereinafter referred to as tandem light-emittingelement) in which a charge generation layer is provided between aplurality of EL layers will be described.

The light-emitting element described in this embodiment is a tandemlight-emitting element including a plurality of EL layers (a first ELlayer 202(1) and a second EL layer 202(2)) between a pair of electrodes(a first electrode 201 and a second electrode 204) as illustrated inFIG. 2A.

In this embodiment, the first electrode 201 functions as an anode, andthe second electrode 204 functions as a cathode. Note that the firstelectrode 201 and the second electrode 204 can have structures similarto those described in Embodiment 1. In addition, all or any of theplurality of EL layers (the first EL layer 202(1) and the second ELlayer 202(2)) may have structures similar to those described inEmbodiment 1. In other words, the structures of the first EL layer202(1) and the second EL layer 202(2) may be the same or different fromeach other and can be similar to those of the EL layers described inEmbodiment 1. Note that the pyrene-based compound which is oneembodiment of the present invention and described in Embodiment 2 can beused for any or all of the plurality of EL layers (the first EL layer202(1) and the second EL layer 202(2)) described in this embodiment.

Further, a charge generation layer (I) 205 is provided between theplurality of EL layers (the first EL layer 202(1) and the second ELlayer 202(2)). The charge generation layer (I) 205 has a function ofinjecting electrons into one of the EL layers and injecting holes intothe other of the EL layers when a voltage is applied between the firstelectrode 201 and the second electrode 204. In this embodiment, when avoltage is applied such that the potential of the first electrode 201 ishigher than that of the second electrode 204, the charge generationlayer (I) 205 injects electrons into the first EL layer 202(1) andinjects holes into the second EL layer 202(2).

Note that in terms of light extraction efficiency, the charge generationlayer (I) 205 preferably has a light-transmitting property with respectto visible light (specifically, the charge generation layer (I) 205preferably has a visible light transmittance of 40% or more). Further,the charge generation layer (I) 205 functions even if it has lowerconductivity than the first electrode 201 or the second electrode 204.

The charge generation layer (I) 205 may have either a structure in whichan electron acceptor (acceptor) is added to an organic compound having ahigh hole-transport property or a structure in which an electron donor(donor) is added to an organic compound having a high electron-transportproperty. Alternatively, both of these structures may be stacked.

In the case where the electron acceptor is added to the organic compoundhaving a high hole-transport property, examples of the organic compoundhaving a high hole-transport property include aromatic amine compoundssuch as NPB, TPD, TDATA, MTDATA, and4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB), and the like. The substances mentioned here aremainly substances that have a hole mobility of 10⁻⁶ cm²/Vs or more. Notethat other than these substances, any organic compound that has aproperty of transporting more holes than electrons may be used.

Examples of the electron acceptor are7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation:F₄-TCNQ), chloranil, oxides of transition metals, and oxides of metalsthat belong to Groups 4 to 8 of the periodic table. Specifically,vanadium oxide, niobium oxide, tantalum oxide, chromium oxide,molybdenum oxide, tungsten oxide, manganese oxide, and rhenium oxide arepreferable because of their high electron-accepting property. Amongthese, molybdenum oxide is especially preferable since it is stable inthe air, has a low hygroscopic property, and is easy to handle.

On the other hand, in the case where the electron donor is added to theorganic compound having a high electron-transport property, examples ofthe organic compound having a high electron-transport property which canbe used are metal complexes having a quinoline skeleton or abenzoquinoline skeleton, such as Alq, Almq₃, BeBq₂, and BAlq, and thelike. Other examples are metal complexes having an oxazole-based orthiazole-based ligand, such as Zn(BOX)₂ and Zn(BTZ)₂. Other than metalcomplexes, PBD, OXD-7, TAZ, BPhen, BCP, or the like can be used. Thesubstances mentioned here are mainly substances that have an electronmobility of 10⁻⁶ cm²/Vs or more. Note that other than these substances,any organic compound that has a property of transporting more electronsthan holes may be used.

Examples of the electron donor which can be used are alkali metals,alkaline earth metals, rare earth metals, metals that belong to Groups 2and 13 of the periodic table, and oxides or carbonates thereof.Specifically, lithium (Li), cesium (Cs), magnesium (Mg), calcium (Ca),ytterbium (Yb), indium (In), lithium oxide, cesium carbonate, and thelike are preferable. An organic compound, such as tetrathianaphthacene,may be used as the electron donor.

Note that forming the charge generation layer (I) 205 by using any ofthe above materials can suppress a drive voltage increase caused by thestack of the EL layers.

Although this embodiment shows the light-emitting element having two ELlayers, the present invention can be similarly applied to alight-emitting element in which n EL layers (n is three or more) arestacked as illustrated in FIG. 2B. In the case where a plurality of ELlayers is included between a pair of electrodes as in the light-emittingelement according to this embodiment, by provision of the chargegeneration layer (I) between the EL layers, light emission in a highluminance region can be obtained with current density kept low. Sincethe current density can be kept low, the element can have a longlifetime. When the light-emitting element is applied to lighting,voltage drop due to resistance of an electrode material can be reduced,thereby achieving homogeneous light emission in a large area. Moreover,it is possible to achieve a light-emitting device which can be driven ata low voltage and has low power consumption.

Note that the structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

Embodiment 4

In this embodiment, a light-emitting device including a light-emittingelement which is one embodiment of the present invention will bedescribed.

The light-emitting device can be either a passive matrix light-emittingdevice or an active matrix light-emitting device. Note that any of thelight-emitting elements described in the other embodiments can beapplied to the light-emitting device described in this embodiment.

In this embodiment, an active matrix light-emitting device is describedwith reference to FIGS. 3A and 3B.

Note that FIG. 3A is a top view illustrating a light-emitting device andFIG. 3B is a cross-sectional view taken along the chain line A-A′ inFIG. 3A. The active matrix light-emitting device according to thisembodiment includes a pixel portion 302 provided over an elementsubstrate 301, a driver circuit portion (a source line driver circuit)303, and driver circuit portions (gate line driver circuits) 304 a and304 b. The pixel portion 302, the driver circuit portion 303, and thedriver circuit portions 304 a and 304 b are sealed between the elementsubstrate 301 and a sealing substrate 306 with a sealant 305.

In addition, there is provided a lead wiring 307 over the elementsubstrate 301. The lead wiring 307 is provided for connecting anexternal input terminal through which a signal (e.g., a video signal, aclock signal, a start signal, and a reset signal) or a potential fromthe outside is transmitted to the driver circuit portion 303 and thedriver circuit portions 304 a and 304 b. Here is shown an example inwhich a flexible printed circuit (FPC) 308 is provided as the externalinput terminal. Although only the FPC 308 is illustrated, this FPC maybe provided with a printed wiring board (PWB). The light-emitting devicein this specification includes, in its category, not only thelight-emitting device itself but also the light-emitting device providedwith the FPC or the PWB.

Next, a cross-sectional structure is described with reference to FIG.3B. The driver circuit portions and the pixel portion are formed overthe element substrate 301; here are illustrated the driver circuitportion 303 which is the source line driver circuit and the pixelportion 302.

The driver circuit portion 303 is an example where a CMOS circuit isformed, which is a combination of an n-channel TFT 309 and a p-channelTFT 310. Note that a circuit included in the driver circuit portion maybe formed using any of various circuits, such as a CMOS circuit, a PMOScircuit, or an NMOS circuit. Although a driver-integrated type in whicha driver circuit is formed over the substrate is described in thisembodiment, the present invention is not limited to this type, and thedriver circuit can be formed outside the substrate.

The pixel portion 302 includes a plurality of pixels each of whichincludes a switching TFT 311, a current control TFT 312, and a firstelectrode (anode) 313 which is electrically connected to a wiring (asource electrode or a drain electrode) of the current control TFT 312.Note that an insulator 314 is formed to cover end portions of the firstelectrode (anode) 313. In this embodiment, the insulator 314 is formedusing a positive photosensitive acrylic resin.

The insulator 314 preferably has a curved surface with curvature at anupper end portion or a lower end portion thereof in order to obtainfavorable coverage by a film which is to be stacked over the insulator314. For example, in the case of using a positive photosensitive acrylicresin as a material for the insulator 314, the insulator 314 preferablyhas a curved surface with a curvature radius (0.2 μm to 3 μm) at theupper end portion. The insulator 314 can be formed using either anegative photosensitive resin or a positive photosensitive resin. It ispossible to use, without limitation to an organic compound, either anorganic compound or an inorganic compound such as silicon oxide orsilicon oxynitride.

An EL layer 315 and a second electrode (cathode) 316 are stacked overthe first electrode (anode) 313. In the EL layer 315, at least alight-emitting layer is provided. The light-emitting layer has thestacked structure described in Embodiment 1. The pyrene-based compounddescribed in Embodiment 2 can be used. Further, in the EL layer 315, ahole-injection layer, a hole-transport layer, an electron-transportlayer, an electron-injection layer, a charge generation layer, and thelike can be provided as appropriate in addition to the light-emittinglayer.

The stacked structure of the first electrode (anode) 313, the EL layer315, and the second electrode (cathode) 316 forms a light-emittingelement 317. For the first electrode (anode) 313, the EL layer 315, andthe second electrode (cathode) 316, the materials described inEmbodiment 1 can be used. Although not illustrated, the second electrode(cathode) 316 is electrically connected to the FPC 308 which is anexternal input terminal.

Although the cross-sectional view of FIG. 3B illustrates only onelight-emitting element 317, a plurality of light-emitting elements isarranged in matrix in the pixel portion 302. Light-emitting elementswhich provide three kinds of light emission (R, G, and B) areselectively formed in the pixel portion 302, whereby a light-emittingdevice capable of full color display can be fabricated. Alternatively, alight-emitting device capable of full color display may be fabricated bya combination with color filters.

Further, the sealing substrate 306 is attached to the element substrate301 with the sealant 305, whereby the light-emitting element 317 isprovided in a space 318 surrounded by the element substrate 301, thesealing substrate 306, and the sealant 305. The space 318 may be filledwith an inert gas (such as nitrogen or argon) or the sealant 305.

An epoxy-based resin is preferably used for the sealant 305. It ispreferable that such a material do not transmit moisture or oxygen asmuch as possible. As the sealing substrate 306, a glass substrate, aquartz substrate, or a plastic substrate formed of fiberglass reinforcedplastic (FRP), polyvinyl fluoride (PVF), a polyester, an acrylic resin,or the like can be used.

As described above, an active matrix light-emitting device can beobtained.

Note that the structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

Embodiment 5

In this embodiment, examples of a variety of electronic devices whichare completed using a light-emitting device will be described withreference to FIGS. 4A to 4D and FIGS. 5A to 5C. The light-emittingdevice is fabricated using a light-emitting element which is oneembodiment of the present invention.

Examples of electronic devices to which the light-emitting device isapplied are television devices (also referred to as TV or televisionreceivers), monitors for computers and the like, cameras such as digitalcameras and digital video cameras, digital photo frames, cellular phones(also referred to as portable telephone devices), portable gamemachines, portable information terminals, audio playback devices, largegame machines such as pin-ball machines, and the like. Specific examplesof these electronic devices are illustrated in FIGS. 4A to 4D.

FIG. 4A illustrates an example of a television device. In a televisiondevice 7100, a display portion 7103 is incorporated in a housing 7101.The display portion 7103 is capable of displaying images, and alight-emitting device can be used for the display portion 7103. Inaddition, here, the housing 7101 is supported by a stand 7105.

The television device 7100 can be operated with an operation switchprovided in the housing 7101 or a separate remote controller 7110. Withoperation keys 7109 of the remote controller 7110, channels and volumecan be controlled and images displayed on the display portion 7103 canbe controlled. Furthermore, the remote controller 7110 may be providedwith a display portion 7107 for displaying data to be output from theremote controller 7110.

Note that the television device 7100 is provided with a receiver, amodem, and the like. With the receiver, general television broadcastingcan be received. Furthermore, when the television device 7100 isconnected to a communication network by wired or wireless connection viathe modem, one-way (from a transmitter to a receiver) or two-way(between a transmitter and a receiver, between receivers, or the like)data communication can be performed.

FIG. 4B illustrates a computer, which includes a main body 7201, ahousing 7202, a display portion 7203, a keyboard 7204, an externalconnection port 7205, a pointing device 7206, and the like. Note thatthis computer is manufactured by using a light-emitting device for thedisplay portion 7203.

FIG. 4C illustrates a portable game machine, which includes twohousings, i.e., a housing 7301 and a housing 7302, connected to eachother via a joint portion 7303 so that the portable game machine can beopened or closed. A display portion 7304 is incorporated in the housing7301 and a display portion 7305 is incorporated in the housing 7302. Inaddition, the portable game machine illustrated in FIG. 4C includes aspeaker portion 7306, a recording medium insertion portion 7307, an LEDlamp 7308, input means (an operation key 7309, a connection terminal7310, a sensor 7311 (a sensor having a function of measuring force,displacement, position, speed, acceleration, angular velocity,rotational frequency, distance, light, liquid, magnetism, temperature,chemical substance, sound, time, hardness, electric field, electriccurrent, voltage, electric power, radiation, flow rate, humidity,gradient, oscillation, odor, or infrared rays), and a microphone 7312),and the like. It is needless to say that the structure of the portablegame machine is not limited to the above structure as long as alight-emitting device is used for at least either the display portion7304 or the display portion 7305, or both, and may include otheraccessories as appropriate. The portable game machine illustrated inFIG. 4C has a function of reading out a program or data stored in astorage medium to display it on the display portion, and a function ofsharing information with another portable game machine by wirelesscommunication. Note that the portable game machine illustrated in FIG.4C can have a variety of functions without limitation to those above.

FIG. 4D illustrates an example of a cellular phone. A cellular phone7400 is provided with a display portion 7402 incorporated in a housing7401, operation buttons 7403, an external connection port 7404, aspeaker 7405, a microphone 7406, and the like. Note that the cellularphone 7400 is manufactured using a light-emitting device for the displayportion 7402.

When the display portion 7402 of the cellular phone 7400 illustrated inFIG. 4D is touched with a finger or the like, data can be input to thecellular phone 7400. Further, operations such as making a call andcreating e-mail can be performed by touch on the display portion 7402with a finger or the like.

There are mainly three screen modes for the display portion 7402. Thefirst mode is a display mode mainly for displaying an image. The secondmode is an input mode mainly for inputting information such ascharacters. The third mode is a display-and-input mode in which twomodes of the display mode and the input mode are mixed.

For example, in the case of making a call or creating e-mail, acharacter input mode mainly for inputting characters is selected for thedisplay portion 7402 so that characters displayed on the screen can beinput. In this case, it is preferable to display a keyboard or numberbuttons on almost the entire screen of the display portion 7402.

When a detection device including a sensor for detecting inclination,such as a gyroscope or an acceleration sensor, is provided inside thecellular phone 7400, display on the screen of the display portion 7402can be automatically changed by determining the orientation of thecellular phone 7400 (whether the cellular phone is placed horizontallyor vertically for a landscape mode or a portrait mode).

The screen modes are changed by touch on the display portion 7402 oroperation with the operation buttons 7403 of the housing 7401.Alternatively, the screen modes can be changed depending on the kind ofimage displayed on the display portion 7402. For example, when a signalfor an image to be displayed on the display portion is data of movingimages, the screen mode is changed to the display mode. When the signalis text data, the screen mode is changed to the input mode.

Moreover, in the input mode, if a signal detected by an optical sensorin the display portion 7402 is detected and the input by touch on thedisplay portion 7402 is not performed for a certain period, the screenmode may be controlled so as to be changed from the input mode to thedisplay mode.

The display portion 7402 may function as an image sensor. For example,an image of a palm print, a fingerprint, or the like is taken by touchon the display portion 7402 with the palm or the finger, wherebypersonal identification can be performed. Furthermore, when a backlightor a sensing light source which emits near-infrared light is providedfor the display portion, an image of a finger vein, a palm vein, or thelike can also be taken.

FIGS. 5A and 5B illustrate a foldable tablet terminal. The tabletterminal is opened in FIG. 5A. The tablet terminal includes a housing9630, a display portion 9631 a, a display portion 9631 b, a display modeswitch 9034, a power switch 9035, a power saver switch 9036, a clasp9033, and an operation switch 9038. The tablet terminal is manufacturedusing the light-emitting device for either the display portion 9631 a orthe display portion 9631 b or both.

Part of the display portion 9631 a can be a touch panel region 9632 aand data can be input when a displayed operation key 9637 is touched.Although a structure in which a half region in the display portion 9631a has only a display function and the other half region also has a touchpanel function is shown as an example, the display portion 9631 a is notlimited to the structure. The whole region in the display portion 9631 amay have a touch panel function. For example, the display portion 9631 acan display keyboard buttons in the whole region to be a touch panel,and the display portion 9631 b can be used as a display screen.

As in the display portion 9631 a, part of the display portion 9631 b canbe a touch panel region 9632 b. When a keyboard display switching button9639 displayed on the touch panel is touched with a finger, a stylus, orthe like, a keyboard can be displayed on the display portion 9631 b.

Touch input can be performed in the touch panel region 9632 a and thetouch panel region 9632 b at the same time.

The display mode switch 9034 can switch the display between portraitmode, landscape mode, and the like, and between monochrome display andcolor display, for example. The power saver switch 9036 can controldisplay luminance in accordance with the amount of external light in useof the tablet terminal detected by an optical sensor incorporated in thetablet terminal. In addition to the optical sensor, another detectiondevice including a sensor for detecting inclination, such as a gyroscopeor an acceleration sensor, may be incorporated in the tablet terminal.

Note that FIG. 5A shows an example in which the display portion 9631 aand the display portion 9631 b have the same display area; however,without limitation thereon, one of the display portions may be differentfrom the other display portion in size and display quality. For example,one display panel may be capable of higher-definition display than theother display panel.

The tablet terminal is closed in FIG. 5B. The tablet terminal includesthe housing 9630, a solar cell 9633, a charge and discharge controlcircuit 9634, a battery 9635, and a DCDC converter 9636. In FIG. 5B, astructure including the battery 9635 and the DCDC converter 9636 isillustrated as an example of the charge and discharge control circuit9634.

Since the tablet terminal is foldable, the housing 9630 can be closedwhen the tablet terminal is not used. As a result, the display portion9631 a and the display portion 9631 b can be protected; thus, a tabletterminal which has excellent durability and excellent reliability interms of long-term use can be provided.

In addition, the tablet terminal illustrated in FIGS. 5A and 5B can havea function of displaying a variety of kinds of data (e.g., a stillimage, a moving image, and a text image), a function of displaying acalendar, a date, the time, or the like on the display portion, atouch-input function of operating or editing the data displayed on thedisplay portion by touch input, a function of controlling processing bya variety of kinds of software (programs), and the like.

The solar cell 9633 provided on a surface of the tablet terminal cansupply power to the touch panel, the display portion, a video signalprocessing portion, or the like. Note that a structure in which thesolar cell 9633 is provided on one or both surfaces of the housing 9630is preferable because the battery 9635 can be charged efficiently. Theuse of a lithium ion battery as the battery 9635 is advantageous indownsizing or the like.

The structure and the operation of the charge and discharge controlcircuit 9634 illustrated in FIG. 5B will be described with reference toa block diagram in FIG. 5C. The solar cell 9633, the battery 9635, theDCDC converter 9636, a converter 9638, switches SW1 to SW3, and thedisplay portion 9631 are illustrated in FIG. 5C, and the battery 9635,the DCDC converter 9636, the converter 9638, and the switches SW1 to SW3correspond to the charge and discharge control circuit 9634 illustratedin FIG. 5B.

First, an example of the operation in the case where power is generatedby the solar cell 9633 using external light is described. The voltage ofpower generated by the solar battery is stepped up or down by the DCDCconverter 9636 so that the power has a voltage for charging the battery9635. Then, when the power from the solar cell 9633 is used for theoperation of the display portion 9631, the switch SW1 is turned on andthe voltage of the power is stepped up or down by the converter 9638 soas to be a voltage needed for the display portion 9631. In addition,when display on the display portion 9631 is not performed, the switchSW1 is turned off and the switch SW2 is turned on so that the battery9635 may be charged.

Note that the solar cell 9633 is described as an example of a powergeneration means; however, without limitation thereon, the battery 9635may be charged using another power generation means such as apiezoelectric element or a thermoelectric conversion element (Peltierelement). For example, a non-contact electric power transmission modulewhich transmits and receives power wirelessly (without contact) tocharge the battery 9635, or a combination of the solar cell 9633 andanother means for charge may be used.

It is needless to say that an embodiment of the present invention is notlimited to the electronic device illustrated in FIGS. 5A to 5C as longas the display portion described in the above embodiment is included.

As described above, the electronic devices can be obtained by the use ofthe light-emitting device which is one embodiment of the presentinvention. The light-emitting device has a remarkably wide applicationrange, and can be applied to electronic devices in a variety of fields.

Note that the structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

Embodiment 6

In this embodiment, examples of lighting devices will be described withreference to FIG. 6. A light-emitting device including a light-emittingelement which is one embodiment of the present invention is applied tothe lighting devices.

FIG. 6 illustrates an example in which a light-emitting device is usedfor an interior lighting device 8001. Since the light-emitting devicecan have a larger area, a lighting device having a large area can alsobe formed. In addition, a lighting device 8002 in which a light-emittingregion has a curved surface can also be formed with the use of a housingwith a curved surface. A light-emitting element included in thelight-emitting device described in this embodiment is in the form of athin film, which allows the housing to be designed more freely.Therefore, the lighting device can be elaborately designed in a varietyof ways. Further, a wall of the room may be provided with a large-sizedlighting device 8003.

Moreover, when the light-emitting device is used for a table by beingused as a surface of a table, a lighting device 8004 which has afunction as a table can be obtained. When the light-emitting device isused as part of other furniture, a lighting device which has a functionas the furniture can be obtained.

In this manner, a variety of lighting devices to which thelight-emitting device is applied can be obtained. Note that suchlighting devices are also embodiments of the present invention.

The structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

Example 1 Synthesis Example 1

In this example, a method of synthesizingN,N′-diphenyl-N,N′-(1,6-pyrenyl)-N,N′-bis(9-phenyl-9H-carbazol-3-yl)diamine(abbreviation: 1,6PCAPrn), the pyrene-based compound which is oneembodiment of the present invention and represented by the structuralformula (100) in Embodiment 2, will be described. Note that a structureof 1,6PCAPrn (abbreviation) is shown below.

Synthesis ofN,N′-Diphenyl-N,N′-(1,6-pyrenyl)-N,N′-bis(9-phenyl-9H-carbazol-3-yl)diamine(Abbreviation: 1,6PCAPrn)

In a 100 mL three-neck flask were placed 0.80 g (2.2 mmol) of1,6-dibromopyrene, 1.5 g (4.4 mmol) ofN-phenyl-N-(9-phenyl-9H-carbazol-3-yl)amine, and 0.86 g (9.0 mmol) ofsodium tert-butoxide. After the air in the flask was replaced withnitrogen, 25 mL of toluene and 2.2 mL of tri-tert-butylphosphine (a 10wt % hexane solution) were added to the mixture.

While the pressure was reduced, this mixture was degassed by beingstirred. After the degassing, 0.12 g (0.22 mmol) ofbis(dibenzylideneacetone)palladium(0) was added to the mixture. Thismixture was stirred at 110° C. for 7 hours under a nitrogen stream, sothat a solid was precipitated. After the stirring, this mixture wassubjected to suction filtration to give a solid.

The obtained solid was dissolved in about 500 mL of hot toluene, andthis solution was subjected to suction filtration through Celite,alumina, and Florisil. The obtained filtrate was concentrated to give asolid. The obtained solid was recrystallized from chloroform/hexane togive 0.84 g of a target yellow powder solid in a yield of 43%.

In addition, by a train sublimation method, 0.84 g of the obtainedyellow powder solid was purified. The sublimation purification wasconducted under the conditions where the pressure was 3.5 Pa, the flowrate of an argon gas was 5.0 mL/min, and the heating temperature was328° C. After the sublimation purification, 0.31 g of a yellow solid of1,6PCAPrn was obtained in a collection rate of 37%.

The reaction scheme of the above synthesis method is illustrated in(a-1) below.

Results of nuclear magnetic resonance spectroscopy (¹H-NMR), by whichthe compound obtained by the above synthesis method was analyzed, areshown below. FIGS. 7A and 7B show the ¹H-NMR charts. Note that FIG. 7Bis an enlarged chart of FIG. 7A. The results reveal thatN,N′-diphenyl-N,N′-(1,6-pyrenyl)-N,N′-bis(9-phenyl-9H-carbazol-3-yl)diamine(abbreviation: 1,6PCAPrn), the pyrene-based compound which is oneembodiment of the present invention and represented by the abovestructural formula (100), was obtained.

¹H NMR (C₂H₄Cl₄, 300 MHz): δ=8.16 (t, J=7.2 Hz, 2H), 8.26 (d, J=8.4 Hz,4H), 8.43-8.52 (m, 9H), 8.59-8.74 (m, 10H), 8.84-8.86 (m, 8H), 9.15-9.22(m, 3H), 9.26 (s, 2H), 9.36 (d, J=8.1 Hz, 2H), 9.54 (d, J=8.7 Hz, 2H).

Next, 1,6PCAPrn (abbreviation) obtained in this example was analyzed byliquid chromatography mass spectrometry (LC/MS).

The analysis by LC/MS was carried out with Acquity UPLC (manufactured byWaters Corporation), and Xevo G2 Tof MS (manufactured by WatersCorporation).

In the MS analysis, ionization was carried out by an electrosprayionization (ESI) method. The capillary voltage and the sample conevoltage were set to 3.0 kV and 30 V, respectively. Detection wasperformed in a positive mode.

A component which underwent the ionization under the above conditionswas collided with an argon gas in a collision cell to dissociate into aplurality of product ions. Energy (collision energy) for the collisionwith argon was 70 eV. The mass range for the measurement was m/z=100 to1200.

FIG. 21 shows the measurement results. The results in FIG. 21 show thatproduct ions of 1,6PCAPrn (abbreviation), the pyrene-based compoundwhich is one embodiment of the present invention and represented by thestructural formula (100), are detected mainly around m/z=790, m/z=624,and m/z=532.

The results in FIG. 21 are characteristically derived from 1,6PCAPrn(abbreviation) and thus can be regarded as important data inidentification of 1,6PCAPrn (abbreviation) contained in a mixture.

Product ions around m/z=790 are presumed to be cations in the statewhere one phenyl group is dissociated from the compound of thestructural formula (100). This is one of features of the pyrene-basedcompound which is one embodiment of the present invention. Product ionsaround m/z=624 are presumed to be cations in the state where one9-phenyl-9H-carbazolyl group is dissociated from the compound of thestructural formula (100), which indicates that the pyrene-based compound1,6PCAPrn (abbreviation) which is one embodiment of the presentinvention includes a 9-phenyl-9H-carbazolyl group. Product ions aroundm/z=381 are presumed to be cations in the state where two phenyl groupsare bound to pyrenediamine.

Next, ultraviolet-visible absorption spectra (hereinafter simplyreferred to as “absorption spectra”) and emission spectra of 1,6PCAPrn(abbreviation) were measured. The absorption spectra were measured usingan ultraviolet-visible light spectrophotometer (V550 type manufacturedby JASCO Corporation). The emission spectra were measured using afluorescence spectrophotometer (FS920 manufactured by HamamatsuPhotonics K.K.). The absorption spectra and the emission spectra of atoluene solution of 1,6PCAPrn (abbreviation) and a thin film of1,6PCAPrn (abbreviation) were measured. Put in a quartz cell, thetoluene solution was subjected to the measurement at room temperature.As for the thin film, the thin film which was deposited on a quartzsubstrate by evaporation was used. In order to obtain the absorptionspectrum of the thin film, an absorption spectrum of quartz wassubtracted from an absorption spectrum of the thin film and quartz.

FIGS. 8A and 8B show measurement results of the absorption spectra andemission spectra. FIG. 8A shows the measurement results of the toluenesolution of 1,6PCAPrn (abbreviation). FIG. 8B shows the measurementresults of the thin film of 1,6PCAPrn (abbreviation). In each of FIGS.8A and 8B, the horizontal axis represents wavelength (nm) and thevertical axis represents absorption intensity (arbitrary unit) oremission intensity (arbitrary unit). In each of FIGS. 8A and 8B, twosolid lines are shown; a thin line represents the absorption spectrum,and a thick line represents the emission spectrum.

In the case of the toluene solution of 1,6PCAPrn (abbreviation), anabsorption peak is observed at around 444 nm as shown in FIG. 8A. In thecase of the thin film of 1,6PCAPrn (abbreviation), an absorption peak isobserved at around 450 nm as shown in FIG. 8B.

Further, in the case of the toluene solution of 1,6PCAPrn(abbreviation), the maximum emission wavelength is 489 nm (excitationwavelength: 445 nm) as shown in FIG. 8A. In the case of the thin film of1,6PCAPrn (abbreviation), the maximum emission wavelength is 526 nm(excitation wavelength: 450 nm) as shown in FIG. 8B.

As described above, 1,6PCAPrn (abbreviation) was found to emitblue-green light with high color purity and accordingly can be used as ablue-green light-emitting material.

Further, the HOMO level and the LUMO level of 1,6PCAPrn (abbreviation)were obtained by cyclic voltammetry (CV) measurement. An electrochemicalanalyzer (ALS model 600A or 600C, manufactured by BAS Inc.) was used forthe CV measurement.

Further, as for the solution used for the CV measurement, dehydrateddimethylformamide (DMF, manufactured by Sigma-Aldrich Inc., 99.8%,Catalog No. 22705-6) was used as a solvent, and tetra-n-butylammoniumperchlorate (n-Bu₄NClO₄, manufactured by Tokyo Chemical Industry Co.,Ltd., Catalog No. T0836), which was a supporting electrolyte, wasdissolved in the solvent such that the concentration oftetra-n-butylammonium perchlorate was 100 mmol/L. Further, the object tobe measured was dissolved in the solvent such that the concentrationthereof was 2 mmol/L. A platinum electrode (PTE platinum electrode,manufactured by BAS Inc.) was used as a working electrode, a platinumelectrode (Pt counter electrode for VC-3 (5 cm), manufactured by BASInc.) was used as an auxiliary electrode, and an Ag/Ag⁺ electrode (RE-7reference electrode for nonaqueous solvent, manufactured by BAS Inc.)was used as a reference electrode. The CV measurement was performedunder the following conditions: room temperature (20° C. to 25° C.) anda scan rate of 0.1 V/sec. Note that the potential energy of thereference electrode with respect to the vacuum level was assumed to be−4.94 eV in this example.

On the assumption that the intermediate potential (the half-wavepotential) between the oxidation peak potential E_(pa) and the reductionpeak potential E_(pc) which are obtained in the CV measurementcorresponds to the HOMO level, the HOMO level of 1,6PCAPrn(abbreviation) was calculated to be −5.32 eV, and the LUMO level of1,6PCAPrn (abbreviation) was calculated to be −2.75 eV.

Note that 1,6PCAPrn (abbreviation) synthesized in this example andanother pyrene-based compound having a different HOMO level but havingsubstantially the same LUMO level are used to form a stack-typelight-emitting layer. The stacked-type light-emitting layer has ahole-trapping property at the interface between the stackedlight-emitting layers but is unlikely to block electron transfer.

Example 2 Synthesis Example 2

In this example, a method of synthesizingN,N′-diphenyl-N,N′-(1,6-pyrenyl)-N,N′-bis[4-(9-phenyl-9H-carbazol-3-yl)phenyl]diamine(abbreviation: 1,6PCBAPrn), the pyrene-based compound which is oneembodiment of the present invention and represented by the structuralformula (108) in Embodiment 2, will be described. Note that a structureof 1,6PCBAPrn (abbreviation) is shown below.

Synthesis ofN,N′-Diphenyl-N,N′-(1,6-pyrenyl)-N,N′-bis[4-(9-phenyl-9H-carbazol-3-yl)phenyl]diamine(Abbreviation: 1,6PCBAPrn)

In a 50 mL three-neck flask were placed 0.4 g (1.2 mmol) of1,6-dibromopyrene, 1.5 g (3.5 mmol) of4-(9-phenyl-9H-carbazol-3-yl)diphenylamine, and 0.5 g (5.3 mmol) ofsodium tert-butoxide. The air in the flask was replaced with nitrogen.

Then, 17.7 mL of toluene and 0.2 mL of a 10% hexane solution oftri(tert-butyl)phosphine were added to this mixture. The temperature ofthis mixture was set to 80° C., 33.4 mg (0.05 mmol) ofbis(dibenzylideneacetone)palladium(0) was added, and the mixture wasstirred for 4.0 hours. Then, 22.4 mg (0.04 mmol) ofbis(dibenzylideneacetone)palladium(0) was added, and the mixture wasstirred for 0.5 hours. After the stirring, the mixture was subjected tosuction filtration through Florisil, Celite, and alumina to give afiltrate.

The filtrate was concentrated to give a solid, which was then purifiedby silica gel column chromatography (the developing solvent has a 3:2ratio of hexane to toluene). The obtained fractions were concentrated togive a target yellow solid. The obtained solid was recrystallized fromchloroform/hexane to give 1.1 g of a yellow solid in a yield of 90%.

By a train sublimation method, 0.8 g of the obtained yellow solid waspurified. The sublimation purification was conducted under theconditions where the pressure was 1.1×10⁻² Pa and the heating conditionswere 400° C. for 4.5 hours and 408° C. for 3.0 hours. After thesublimation purification, 0.4 g of a target yellow solid was obtained ina collection rate of 48%.

The reaction scheme of the above synthesis method is illustrated in(b-1) below.

Results of nuclear magnetic resonance spectroscopy (¹H-NMR), by whichthe compound obtained by the above synthesis method was analyzed, areshown below. FIGS. 9A and 9B show the ¹H-NMR charts. Note that FIG. 9Bis an enlarged chart of FIG. 9A. The results reveal thatN,N′-diphenyl-N,N′-(1,6-pyrenyl)-N,N′-bis[4-(9-phenyl-9H-carbazol-3-yl)phenyl]diamine(abbreviation: 1,6PCBAPm), the pyrene-based compound which is oneembodiment of the present invention and represented by the abovestructural formula (108), was obtained.

¹H NMR (CHCl₃, 300 MHz): δ=6.98 (t, J=7.2 Hz, 2H), 7.14-7.31 (m, 14H),7.38-7.50 (m, 8H), 7.56-7.64 (m, 14H), 7.89 (d, J=7.8 Hz, 2H), 7.97 (d,J=9.3 Hz, 2H), 8.13-8.17 (m, 4H), 8.21 (d, J=9.3 Hz, 2H), 8.31 (d, J=2.1Hz, 2H).

Next, ultraviolet-visible absorption spectra (hereinafter simplyreferred to as “absorption spectra”) and emission spectra of 1,6PCBAPrn(abbreviation) were measured. The absorption spectra were measured usingan ultraviolet-visible light spectrophotometer (V550 type manufacturedby JASCO Corporation). The emission spectra were measured using afluorescence spectrophotometer (FS920 manufactured by HamamatsuPhotonics K.K.). The absorption spectra and the emission spectra of atoluene solution of 1,6PCBAPrn (abbreviation) and a thin film of1,6PCBAPrn (abbreviation) were measured. Put in a quartz cell, thetoluene solution was subjected to the measurement at room temperature.As for the thin film, the thin film which was deposited on a quartzsubstrate by evaporation was used. In order to obtain the absorptionspectrum of the thin film, an absorption spectrum of quartz wassubtracted from an absorption spectrum of the thin film and quartz.

FIGS. 10A and 10B show measurement results of the absorption spectra andemission spectra. FIG. 10A shows the measurement results of the toluenesolution of 1,6PCBAPrn (abbreviation). FIG. 10B shows the measurementresults of the thin film of 1,6PCBAPrn (abbreviation). In each of FIGS.10A and 10B, the horizontal axis represents wavelength (nm) and thevertical axis represents absorption intensity (arbitrary unit) oremission intensity (arbitrary unit). In each of FIGS. 10A and 10B, twosolid lines are shown; a thin line represents the absorption spectrum,and a thick line represents the emission spectrum.

In the case of the toluene solution of 1,6PCBAPrn (abbreviation), anabsorption peak is observed at around 439 nm as shown in FIG. 10A. Inthe case of the thin film of 1,6PCBAPrn (abbreviation), an absorptionpeak is observed at around 448 nm as shown in FIG. 10B.

Further, in the case of the toluene solution of 1,6PCBAPrn(abbreviation), the maximum emission wavelength is 474 nm (excitationwavelength: 370 nm) as shown in FIG. 10A. In the case of the thin filmof 1,6PCBAPrn (abbreviation), the maximum emission wavelength is 526 nm(excitation wavelength: 438 nm) as shown in FIG. 10B.

As described above, 1,6PCBAPrn (abbreviation) was found to emitblue-green light with high color purity and accordingly can be used as ablue-green light-emitting material.

Further, the HOMO level and the LUMO level of 1,6PCBAPrn (abbreviation)were obtained by cyclic voltammetry (CV) measurement. An electrochemicalanalyzer (ALS model 600A or 600C, manufactured by BAS Inc.) was used forthe CV measurement.

Further, as for the solution used for the CV measurement, dehydrateddimethylformamide (DMF, manufactured by Sigma-Aldrich Inc., 99.8%,Catalog No. 22705-6) was used as a solvent, and tetra-n-butylammoniumperchlorate (n-Bu₄NClO₄, manufactured by Tokyo Chemical Industry Co.,Ltd., Catalog No. T0836), which was a supporting electrolyte, wasdissolved in the solvent such that the concentration oftetra-n-butylammonium perchlorate was 100 mmol/L. Further, the object tobe measured was dissolved in the solvent such that the concentrationthereof was 2 mmol/L. A platinum electrode (PTE platinum electrode,manufactured by BAS Inc.) was used as a working electrode, a platinumelectrode (Pt counter electrode for VC-3 (5 cm), manufactured by BASInc.) was used as an auxiliary electrode, and an Ag/Ag⁺ electrode (RE-7reference electrode for nonaqueous solvent, manufactured by BAS Inc.)was used as a reference electrode. The CV measurement was performedunder the following conditions: room temperature (20° C. to 25° C.) anda scan rate of 0.1 V/sec. Note that the potential energy of thereference electrode with respect to the vacuum level was assumed to be−4.94 eV in this example.

On the assumption that the intermediate potential (the half-wavepotential) between the oxidation peak potential E_(pa) and the reductionpeak potential E_(pc) which are obtained in the CV measurementcorresponds to the HOMO level, the HOMO level of 1,6PCBAPrn(abbreviation) was calculated to be −5.19 eV, and the LUMO level of1,6PCBAPrn (abbreviation) was calculated to be −2.62 eV.

Note that 1,6PCBAPrn (abbreviation) synthesized in this example andanother pyrene-based compound having a different HOMO level but havingsubstantially the same LUMO level are used to form a stack-typelight-emitting layer. The stack-type light-emitting layer has ahole-trapping property at the interface between the stackedlight-emitting layers but is unlikely to block electron transfer.

Example 3

In this example, a light-emitting element 1 including a light-emittinglayer formed using the pyrene-based compoundN,N′-diphenyl-N,N′-(1,6-pyrenyl)-N,N′-bis(9-phenyl-9H-carbazol-3-yl)diamine(1,6PCAPrn (abbreviation) (the structural formula (100)) will bedescribed with reference to FIG. 11. Chemical formulae of materials usedin this example are shown below.

Fabrication of Light-Emitting Element 1

First, a film of indium tin oxide containing silicon oxide (ITSO) wasformed over a glass substrate 1100 by a sputtering method, so that afirst electrode 1101 functioning as an anode was formed. Note that thethickness was set to 110 nm and the electrode area was set to 2 mm×2 mm.

Next, as pretreatment for forming the light-emitting element 1 over thesubstrate 1100, UV ozone treatment was performed for 370 seconds afterwashing of a surface of the substrate with water and baking that wasperformed at 200° C. for one hour.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and was subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus, and then thesubstrate 1100 was cooled down for about 30 minutes.

Next, the substrate 1100 was fixed to a substrate holder in the vacuumevaporation apparatus so that a surface on which the first electrode1101 was provided faced downward. In this example, a case is describedin which a hole-injection layer 1111, a hole-transport layer 1112, alight-emitting layer 1113, an electron-transport layer 1114, and anelectron-injection layer 1115 which are included in an EL layer 1102 aresequentially formed.

The pressure in the vacuum evaporation apparatus was reduced to about10⁻⁴ Pa. Then, 1,3,5-tri(dibenzothiophen-4-yl)benzene (abbreviation:DBT3P-II) and molybdenum(VI) oxide were co-evaporated with a mass ratioof DBT3P-II (abbreviation) to molybdenum oxide being 4:2, whereby thehole-injection layer 1111 was formed over the first electrode 1101. Thethickness of the hole-injection layer 1111 was set to 70 nm. Note that aco-evaporation method is an evaporation method by which a plurality ofdifferent substances is concurrently vaporized from respective differentevaporation sources.

Next, the hole-transport layer 1112 was formed by evaporation of9-phenyl-3-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation:PCzPA) to a thickness of 30 nm.

Next, the light-emitting layer 1113 was formed over the hole-transportlayer 1112. Note that the light-emitting layer 1113 in this example hasa structure in which two layers, a first light-emitting layer 1113 a anda second light-emitting layer 1113 b, are stacked. First, the firstlight-emitting layer 1113 a was formed to a thickness of 5 nm byco-evaporation of 9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviation: CzPA) andN,N′-bis(3-methylphenyl)-N,N′-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]-pyrene-1,6-diamine(abbreviation: 1,6mMemFLPAPrn) with a mass ratio of CzPA (abbreviation)to 1,6mMemFLPAPrn (abbreviation) being 1:0.05. Then, the secondlight-emitting layer 1113 b was formed to a thickness of 20 nm byco-evaporation of 9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviation: CzPA) andN,N′-diphenyl-N,N′-(1,6-pyrenyl)-N,N′-bis(9-phenyl-9H-carbazol-3-yl)diamine(abbreviation: 1,6PCAPrn) with a mass ratio of CzPA (abbreviation) to1,6PCAPrn (abbreviation) being 1:0.1. Thus, the light-emitting layer1113 was formed.

Next, over the light-emitting layer 1113, the electron-transport layer1114 was formed in such a manner that a film of9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: CzPA) wasformed by evaporation to a thickness of 10 nm and then a film ofbathophenanthroline (abbreviation: Bphen) was formed by evaporation to athickness of 15 nm. Further, over the electron-transport layer 1114, afilm of lithium fluoride was formed by evaporation to a thickness of 1nm to form the electron-injection layer 1115.

Lastly, over the electron-injection layer 1115, an aluminum film wasformed by evaporation to a thickness of 200 nm as a second electrode1103 functioning as a cathode. Thus, the light-emitting element 1 wasfabricated. Note that, in all the above evaporation steps, evaporationwas performed by a resistance-heating method.

Table 1 shows an element structure of the light-emitting element 1obtained as described above.

TABLE 1 Hole- Hole- Electron- First injection transport Electron-injection Second electrode layer layer Light-emitting layer transportlayer layer electrode Light- ITSO DBT3P-II:MoOx PCzPA CzPA:1, CzPA:1,CzPA Bphen LiF Al emitting (110 nm) (4:2, 70 nm) (30 nm) 6mMemFLPAPrn6mPCAPrn (10 nm) (15 nm) (1 nm) (200 nm) element 1 (1:0.05, 5 nm)(1:0.1, 20 nm)

The fabricated light-emitting element 1 was sealed in a glove boxcontaining a nitrogen atmosphere so as not to be exposed to air.

Operation Characteristics of Light-Emitting Element 1)

Operation characteristics of the fabricated light-emitting element 1were measured. Note that the measurements were carried out at roomtemperature (in an atmosphere kept at 25° C.).

FIG. 12 shows luminance-current efficiency characteristics of thelight-emitting element 1. In FIG. 12, the vertical axis representscurrent efficiency (cd/A), and the horizontal axis represents luminance(cd/m²). FIG. 13 shows voltage-current characteristics of thelight-emitting element 1. Note that in FIG. 13, the vertical axisrepresents current (mA), and the horizontal axis represents voltage (V).FIG. 14 shows the CIE chromaticity coordinates of the light-emittingelement 1. Note that in FIG. 14, the vertical axis represents they-coordinate, and the horizontal axis represents the x-coordinate. Table2 below shows initial values of main characteristics of thelight-emitting element 1 at a luminance of about 1000 cd/m².

TABLE 2 Current Current Power External Voltage Current densityChromaticity Luminance efficiency efficiency quantum (V) (mA) (mA/cm²)(x, y) (cd/m²) (cd/A) (lm/W) efficiency (%) Light-emitting 3.1 0.28 7.0(0.16, 0.33) 1200 17 17 8.5 element 1

The above results show that the light-emitting element 1 fabricated inthis example has high external quantum efficiency, which means its highemission efficiency. Moreover, as for color purity, it can be found thatthe light-emitting element exhibits blue-green emission with excellentcolor purity.

FIG. 15 shows an emission spectrum when a current at a current densityof 25 mA/cm² was supplied to the light-emitting element 1. FIG. 15 showsthat the emission spectrum of the light-emitting element 1 has peaks ataround 491 nm and 470 nm, which indicates that the emission spectrum isderived from emission from the pyrene-based compounds 1,6PCAPrn(abbreviation) and 1,6mMemFLPAPrn (abbreviation).

The light-emitting element 1 was subjected to a reliability test. Theresults are shown in FIG. 16. Note that in the reliability test, thelight-emitting element 1 was driven under the conditions where theinitial luminance was set to 5000 cd/m² and the current density wasconstant. As a result, the light-emitting element 1 kept about 74% ofthe initial luminance after 300 hours elapsed. Thus, the reliabilitytest revealed high reliability of the light-emitting element 1.

The light-emitting element described in this example emits blue lightoriginating from 1,6mMemFLPAPrn (abbreviation) and blue-green lightoriginating from 1,6PCAPrn (abbreviation) which are guest materialscontained in the light-emitting layer. In general, the external quantumefficiency (7.2%) of a light-emitting element which contains 1,6PCAPrn(abbreviation) as a guest material and emits blue-green light tends tobe lower than the external quantum efficiency (8.4%) of a light-emittingelement which contains 1,6mMemFLPAPrn (abbreviation) as a guest materialand emits blue light. In the case of this example, even thoughcontaining both the substances as guest materials, the light-emittingelement has an external quantum efficiency of 8.5% which is higher orsubstantially the same.

Therefore, as described in this example, a light-emitting element isformed which includes stacked light-emitting layers containing a commonhost material and different pyrene-based compounds having different HOMOlevels but having substantially the same LUMO levels (1,6PCBAPrn(abbreviation) with a HOMO level of −5.32 eV and a LUMO level of −2.75eV and 1,6mMemFLPAPrn (abbreviation) with a HOMO level of −5.50 eV and aLUMO level of −2.82 eV) as guest materials. Accordingly, a stack-typelight-emitting layer can be formed which has a hole-trapping property atthe interface between the stacked light-emitting layers due to thedifference between the HOMO levels of the guest materials, but isunlikely to block electron transfer because the guest materials havesubstantially the same LUMO levels. Thus, a light-emitting elementhaving high emission efficiency can be obtained. The common hostmaterial is preferably a material containing anthracene, particularly, amaterial containing anthracene and having no amine skeleton, such asCzPA (with a HOMO level of −5.64 eV and a LUMO level of −2.71 eV) usedin this example. Thus, the light-emitting element can also have longlifetime. Furthermore, when another light-emitting layer which emitslight of a different color is additionally stacked in the light-emittingelement having the above structure, the light-emitting element can havebetter color rendering properties.

Reference Example

In this reference example, a synthesis example ofN,N′-bis(3-methylphenyl)-N,N′-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]-pyrene-1,6-diamine(abbreviation: 1,6mMemFLPAPrn) used in Example 3 and represented by thefollowing structural formula will be described.

Step 1: Method of Synthesizing3-Methylphenyl-3-(9-phenyl-9H-fluoren-9-yl)phenylamine (Abbreviation:mMemFLPA)

In a 200 mL three-neck flask were placed 3.2 g (8.1 mmol) of9-(3-bromophenyl)-9-phenylfluorene and 2.3 g (24.1 mmol) of sodiumtert-butoxide. The air in the flask was replaced with nitrogen. To thismixture were added 40.0 mL of toluene, 0.9 mL (8.3 mmol) of m-toluidine,and 0.2 mL of a 10% hexane solution of tri(tert-butyl)phosphine. Thetemperature of this mixture was set to 60° C., 44.5 mg (0.1 mmol) ofbis(dibenzylideneacetone)palladium(0) was added to the mixture, and thetemperature of this mixture was set to 80° C., followed by stirring for2.0 hours. After the stirring, the mixture was subjected to suctionfiltration through Florisil, Celite, and alumina to give a filtrate. Theobtained filtrate was concentrated to give a solid, which was thenpurified by silica gel column chromatography (the developing solvent hasa 1:1 ratio of hexane to toluene) and recrystallized from a mixedsolvent of toluene and hexane. Accordingly, 2.8 g of a target whitesolid was obtained in a yield of 82%. The synthesis scheme of Step 1 isillustrated in (C-1) below.

[Step 2: Method of SynthesizingN,N′-Bis(3-methylphenyl)-N,N′-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]-pyrene-1,6-diamine(Abbreviation: 1,6mMemFLPAPrn)]

In a 100 mL three-neck flask were placed 0.6 g (1.7 mmol) of1,6-dibromopyrene, 1.4 g (3.4 mmol) of3-methylphenyl-3-(9-phenyl-9H-fluoren-9-yl)phenylamine, and 0.5 g (5.1mmol) of sodium tert-butoxide. The air in the flask was replaced withnitrogen. Then, 21.0 mL of toluene and 0.2 mL of a 10% hexane solutionof tri(tert-butyl)phosphine were added to this mixture. The temperatureof this mixture was set to 60° C., 34.9 mg (0.1 mmol) ofbis(dibenzylideneacetone)palladium(0) was added to the mixture, and thetemperature of this mixture was set to 80° C., followed by stirring for3.0 hours. After the stirring, 400 mL of toluene was added to themixture, and the mixture was heated. While the mixture was kept hot, itwas subjected to suction filtration through Florisil, Celite, andalumina to give a filtrate. The filtrate was concentrated to give asolid, which was then purified by silica gel column chromatography (thedeveloping solvent has a 3:2 ratio of hexane to toluene) to give ayellow solid. The obtained yellow solid was recrystallized from a mixedsolvent of toluene and hexane to give 1.2 g of a target yellow solid ina yield of 67%.

By a train sublimation method, 1.0 g of the obtained yellow solid waspurified. The sublimation purification was conducted under theconditions where the pressure was 2.2 Pa, the flow rate of an argon gaswas 5.0 mL/min, and the yellow solid was heated at 317° C. After thesublimation purification, 1.0 g of a target yellow solid was obtained ina collection rate of 93%. The synthesis scheme of Step 2 is illustratedin (C-2) below.

The compound was identified asN,N′-bis(3-methylphenyl)-N,N′-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]-pyrene-1,6-diamine(abbreviation: 1,6mMemFLPAPrn), which was the target substance of thesynthesis, by nuclear magnetic resonance (NMR) spectroscopy.

¹H NMR data of the obtained compound are as follows: ¹H NMR (CDCl₃, 300MHz): δ=2.21 (s, 6H), 6.67 (d, J=7.2 Hz, 2H), 6.74 (d, J=7.2 Hz, 2H),7.17-7.23 (m, 34H), 7.62 (d, J=7.8 Hz, 4H), 7.74 (d, J=7.8 Hz, 2H), 7.86(d, J=9.0 Hz, 2H), 8.04 (d, J=8.7 Hz, 4H).

Example 4

In this example, a light-emitting element 2 illustrated in FIG. 17 andincluding a light-emitting layer formed using the pyrene-based compoundsynthesized in Example 1,N,N′-diphenyl-N,N′-(1,6-pyrenyl)-N,N′-bis(9-phenyl-9H-carbazol-3-yl)diamine(abbreviation: 1,6PCAPrn) (the structural formula (100)), wasfabricated, and its operation characteristics were measured. Note thatthe light-emitting element 2 fabricated in this example is alight-emitting element (hereinafter referred to as tandem light-emittingelement) in which a charge generation layer is provided between aplurality of EL layers as described in Embodiment 3. Chemical formulaeof materials used in this example are shown below.

Fabrication of Light-Emitting Element

First, a film of indium tin oxide containing silicon oxide (ITSO) wasformed over a glass substrate 3000 by a sputtering method, so that afirst electrode 3001 functioning as an anode was formed. Note that thethickness was set to 110 nm and the electrode area was set to 2 mm×2 mm.

Next, as pretreatment for forming the light-emitting element 2 over thesubstrate 3000, UV ozone treatment was performed for 370 seconds afterwashing of a surface of the substrate with water and baking that wasperformed at 200° C. for one hour.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and was subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus, and then thesubstrate 3000 was cooled down for about 30 minutes.

Next, the substrate 3000 was fixed to a substrate holder in the vacuumevaporation apparatus so that a surface on which the first electrode3001 was provided faced downward. In this example, a case is describedin which a first hole-injection layer 3011 a, a first hole-transportlayer 3012 a, a first light-emitting layer 3013 a, a firstelectron-transport layer 3014 a, and a first electron-injection layer3015 a which are included in a first EL layer 3002 a are sequentiallyformed, a charge generation layer 3016 is formed, and then a secondhole-injection layer 3011 b, a second hole-transport layer 3012 b, asecond light-emitting layer 3013 b, a second electron-transport layer3014 b, and a second electron-injection layer 3015 b which are includedin a second EL layer 3002 b are formed.

The pressure in the vacuum evaporation apparatus was reduced to about10⁻⁴ Pa. Then, 9-[4-(9-phenylcarbazol-3-yl)]phenyl-10-phenylanthracene(abbreviation: PCzPA) and molybdenum(VI) oxide were co-evaporated with amass ratio of PCzPA (abbreviation) to molybdenum oxide being 1:0.5,whereby the first hole-injection layer 3011 a was formed over the firstelectrode 3001. The thickness of the first hole-injection layer 3011 awas set to 33.3 nm. Note that a co-evaporation method is an evaporationmethod by which a plurality of different substances is concurrentlyvaporized from respective different evaporation sources.

Next, the first hole-transport layer 3012 a was formed by evaporation ofPCzPA (abbreviation) to a thickness of 30 nm.

Next, the first light-emitting layer 3013 a was formed over the firsthole-transport layer 3012 a. A film was formed to a thickness of 5 nm byco-evaporation of 9-[4-(10-phenyl-9-anthracenyl)phenyl]-9H-carbazole(abbreviation: CzPA) andN,N′-bis(3-methylphenyl)-N,N′-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]-pyrene-1,6-diamine(abbreviation: 1,6mMemFLPAPrn) with a mass ratio of CzPA (abbreviation)to 1,6mMemFLPAPrn (abbreviation) being 1:0.05. Then, a film was formedthereover to a thickness of 25 nm by co-evaporation of CzPA(abbreviation) andN,N′-diphenyl-N,N′-(1,6-pyrenyl)-N,N′-bis(9-phenyl-9H-carbazol-3-yl)diamine(abbreviation: 1,6PCAPrn) with a mass ratio of CzPA (abbreviation) to1,6PCAPrn (abbreviation) being 1:0.01. Thus, the first light-emittinglayer 3013 a was formed.

Next, over the first light-emitting layer 3013 a, the firstelectron-transport layer 3014 a was formed in such a manner that a filmof CzPA (abbreviation) was formed by evaporation to a thickness of 5 nmand then a film of bathophenanthroline (abbreviation: Bphen) was formedby evaporation to a thickness of 15 nm. Further, over the firstelectron-transport layer 3014 a, a film of lithium oxide (Li₂O) wasformed by evaporation to a thickness of 0.1 nm to form the firstelectron-injection layer 3015 a.

Then, copper phthalocyanine (abbreviation: CuPc) was evaporated to athickness of 2 nm over the first electron-injection layer 3015 a,whereby the charge generation layer 3016 was formed.

Then, PCzPA (abbreviation) and molybdenum(VI) oxide were co-evaporatedwith a mass ratio of PCzPA (abbreviation) to molybdenum oxide being1:0.5, whereby the second hole-injection layer 3011 b was formed overthe charge generation layer 3016. The thickness of the secondhole-injection layer 3011 b was set to 50 nm.

Next, the second hole-transport layer 3012 b was formed by evaporationof 4-phenyl-4′-(9-phenylfluorene-9-yl)triphenylamine (abbreviation:BPAFLP) to a thickness of 20 nm.

Next, the second light-emitting layer 3013 b was formed over the secondhole-transport layer 3012 b. The second light-emitting layer 3013 b wasformed by co-evaporation of2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:2mDBTPDBq-II), 4-phenyl-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBA1BP), and(acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III)(abbreviation: [Ir(dppm)₂(acac)]) with a mass ratio of 2mDBTPDBq-II(abbreviation) to PCBA1BP (abbreviation) and [Ir(dppm)₂(acac)](abbreviation) being 0.8:0.2:0.06. The thickness of the secondlight-emitting layer 3013 b was set to 40 nm.

Next, over the second light-emitting layer 3013 b, the secondelectron-transport layer 3014 b was formed in such a manner that a filmof 2mDBTPDBq-II (abbreviation) was formed by evaporation to a thicknessof 15 nm and then a film of Bphen (abbreviation) was formed byevaporation to a thickness of 15 nm. Further, over the secondelectron-transport layer 3014 b, a film of lithium fluoride (LiF) wasformed by evaporation to a thickness of 0.1 nm, whereby the secondelectron-injection layer 3015 b was formed.

Lastly, over the second electron-injection layer 3015 b, an aluminumfilm was formed by evaporation to a thickness of 200 nm as a secondelectrode 3003 functioning as a cathode. Thus, the light-emittingelement 2 was fabricated. Note that, in all the above evaporation steps,evaporation was performed by a resistance-heating method.

Table 3 shows an element structure of the light-emitting element 2obtained as described above.

TABLE 3 First First First Charge First First hole- transport emittingFirst electron- injection generation electrode injection layer layerlayer transport layer layer layer Light- ITSO PCzPA:MoOx PCzPA * CzPABphen Li₂O CuPc emitting (110 nm) (1:0.5, 33.3 nm) (30 nm) (5 nm) (15nm) (0.1 nm) (2 nm) element 2 Second Second Second hole- light-electron- Second hole- transport emitting Second electron- injectionSecond injection layer layer layer transport layer layer electrodeLight- PCzPA:MoOx BPAFLP ** 2mDBTPDBq-II Bphen LiF Al emitting (1:0.5,50 nm) (20 nm) (15 nm) (15 nm) (l nm) (200 nm) element 2 *CzPA: 1.6mMemFLPAPrn (1:0.05, 5 nm)\CzPA: 1.6 PCAPrn (1:0.01, 25 nm) **2mDBTPDBq-II: PCBA1BP:[Ir(dppm)₂(acac)] (0.8:0.2:0.06, 40 nm)

The fabricated light-emitting element 2 was sealed in a glove boxcontaining a nitrogen atmosphere so as not to be exposed to air(specifically, a sealant was applied to an outer edge of the element andheat treatment was performed at 80° C. for 1 hour at the time ofsealing).

Operation Characteristics of Light-Emitting Element 2

Operation characteristics of the fabricated light-emitting element 2were measured. Note that the measurements were carried out at roomtemperature (in an atmosphere kept at 25° C.).

FIG. 18 shows luminance-current efficiency characteristics of thelight-emitting element 2. Note that in FIG. 18, the vertical axisrepresents current efficiency (cd/A), and the horizontal axis representsluminance (cd/m²). FIG. 19 shows the CIE chromaticity coordinates of thelight-emitting element 2. Note that in FIG. 19, the vertical axisrepresents the y-coordinate, and the horizontal axis represents thex-coordinate. Table 4 below shows initial values of main characteristicsof the light-emitting element 2 at a luminance of about 1000 cd/m².

TABLE 4 Current Power External Voltage Current density ChromaticityLuminance efficiency quantum (V) (mA) (mA/cm²) (x, y) (cd/m²) (lm/W)efficiency (%) Light- 5.6 0.041 1.03 (0.47, 0.41) 840 46 31 emittingelement 2

The above results show that the light-emitting element 2 fabricated inthis example has high external quantum efficiency, which means its highemission efficiency. Moreover, the chromaticity (x, y) shows that thelight-emitting element 2 emits white light (incandescent color).

FIG. 20 shows an emission spectrum when a current at a current densityof 0.1 mA/cm² was supplied to the light-emitting element 2. FIG. 20shows that the emission spectrum of the light-emitting element 2 haspeaks at around 471 nm and 581 nm, which indicates that the emissionspectrum is derived from emission from the pyrene-based compounds1,6PCAPrn (abbreviation) and 1,6mMemFLPAPrn (abbreviation) and thephosphorescent organometallic iridium complex [Ir(dppm)₂(acac)](abbreviation) contained in the light-emitting layers. Note that ageneral color rendering index (Ra) which is calculated from thisspectrum is 43.6, which means good color rendering properties.

Therefore, as described in this example, a light-emitting element isformed which includes stacked light-emitting layers containing a commonhost material and different pyrene-based compounds having different HOMOlevels but having substantially the same LUMO levels as guest materials.Accordingly, a stack-type light-emitting layer can be formed which has ahole-trapping property at the interface between the stackedlight-emitting layers due to the difference between the HOMO levels ofthe guest materials, but is unlikely to block electron transfer becausethe guest materials have substantially the same LUMO levels. Thus, alight-emitting element having high emission efficiency can be obtained.Furthermore, in the light-emitting element described in this example,the light-emitting layer containing the phosphorescent organometalliciridium complex and emitting light of a different color is stacked.Thus, a white light-emitting element having still higher emissionefficiency can be obtained.

This application is based on Japanese Patent Application serial no.2011-223634 filed with Japan Patent Office on Oct. 11, 2011, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A light-emitting element comprising: a firstelectrode; a first light-emitting layer over the first electrode; asecond light-emitting layer over the first light-emitting layer; and asecond electrode over the second light-emitting layer, wherein the firstlight-emitting layer comprises a bipolar material, a first pyrene-basedcompound, and a second pyrene-based compound, and wherein the secondlight-emitting layer comprises a phosphorescent organometallic iridiumcomplex.
 2. The light-emitting element according to claim 1, wherein aLUMO level of the first pyrene-based compound and a LUMO level of thesecond pyrene-based compound are within a range of ±0.2 eV.
 3. Thelight-emitting element according to claim 1, wherein a LUMO level of thefirst pyrene-based compound and a LUMO level of the second pyrene-basedcompound are within a range of ±0.1 eV.
 4. The light-emitting elementaccording to claim 1, wherein the bipolar material comprises anthracene,pyrene, phenanthrene, triphenylene, or fluoranthene.
 5. Thelight-emitting element according to claim 1, wherein the secondlight-emitting layer comprises the phosphorescent organometallic iridiumcomplex, a first organic compound, and a second organic compound.
 6. Alight-emitting device comprising the light-emitting element according toclaim
 1. 7. A light-emitting element comprising: a first electrode; afirst light-emitting layer over the first electrode; a secondlight-emitting layer over the first light-emitting layer; and a secondelectrode over the second light-emitting layer, wherein the firstlight-emitting layer comprises a material comprising a condensedaromatic hydrocarbon, a first pyrene-based compound, and a secondpyrene-based compound, and wherein the second light-emitting layercomprises a phosphorescent organometallic iridium complex.
 8. Thelight-emitting element according to claim 7, wherein a LUMO level of thefirst pyrene-based compound and a LUMO level of the second pyrene-basedcompound are within a range of ±0.2 eV.
 9. The light-emitting elementaccording to claim 7, wherein a LUMO level of the first pyrene-basedcompound and a LUMO level of the second pyrene-based compound are withina range of ±0.1 eV.
 10. The light-emitting element according to claim 7,wherein the material comprising the condensed aromatic hydrocarboncomprises anthracene, pyrene, phenanthrene, triphenylene, orfluoranthene.
 11. The light-emitting element according to claim 7,wherein the second light-emitting layer comprises the phosphorescentorganometallic iridium complex, a first organic compound, and a secondorganic compound.
 12. A light-emitting device comprising thelight-emitting element according to claim
 7. 13. A light-emittingelement comprising: a first electrode; a first light-emitting layer overthe first electrode; a charge generation layer over the firstlight-emitting layer; a second light-emitting layer over the chargegeneration layer; and a second electrode over the second light-emittinglayer, wherein the first light-emitting layer comprises a materialcomprising a condensed aromatic hydrocarbon, a first pyrene-basedcompound, and a second pyrene-based compound, and wherein the secondlight-emitting layer comprises a phosphorescent organometallic iridiumcomplex.
 14. The light-emitting element according to claim 13, wherein aLUMO level of the first pyrene-based compound and a LUMO level of thesecond pyrene-based compound are within a range of ±0.2 eV.
 15. Thelight-emitting element according to claim 13, wherein a LUMO level ofthe first pyrene-based compound and a LUMO level of the secondpyrene-based compound are within a range of ±0.1 eV.
 16. Thelight-emitting element according to claim 13, wherein the materialcomprising the condensed aromatic hydrocarbon comprises anthracene,pyrene, phenanthrene, triphenylene, or fluoranthene.
 17. Thelight-emitting element according to claim 13, wherein the secondlight-emitting layer comprises the phosphorescent organometallic iridiumcomplex, a first organic compound, and a second organic compound.
 18. Alight-emitting device comprising the light-emitting element according toclaim 13.